1. Field of the Invention
The present invention relates to a numerical control apparatus for controlling a chopping operation of a tool used in a grinding machine and the like, and more specifically, to a numerical control apparatus by which a lower dead point and an upper dead point can be changed.
2. Description of the Related Art
FIG. 3 is a diagram showing an example of a chopping process, wherein a grinding tool 10 is moved upward and downward between a lower dead point Zl and an upper dead point Zu while being rotated about a Z-axis at a cycle of several times per second, and X- and Y-axes are moved accordingly to thereby cause grinding of side surfaces 11a, 11b, . . . of a workpiece 11.
Heretofore, the lower and upper dead points were set by a limit switch or the like, but currently they are set by a program controlled by a numerical control apparatus and can be changed in accordance with the configuration of a workpiece while a chopping operation is effected or performed. The positions of the lower and the upper dead points, a feed speed of chopping, and the like, for example, are changed by instructions from the program. Note that, in some cases, a chopping width is changed instead of the position of the upper dead point.
FIG. 4 is a graph showing a chopping operation carried out in accordance with the instructions issued by the program as described above, wherein a vertical axis represents a Z-axis coordinate and a horizontal axis represents time.
First, at a time t11, the Z-axis is moved at a high speed to a coordinate value Zr of a reference point, and chopping is then started at predetermined intervals using a lower dead point Zl1 and an upper dead point Zu1 output by the program as target values. During the chopping, however, an instructed feed speed is multiplied by an accelerating or decelerating speed having a predetermined time constant and a servo circuit provides a delay. Therefore the Z-axis moves to a next upper dead point before reaching the instructed lower dead point Zl1 or upper dead point Zu1, and thus the actual distance moved is not sufficient.
Therefore, the numerical control apparatus measures a difference between the coordinate value output by the program and the actual position of the Z-axis, and corrects the distance. More specifically, after the chopping is started, instruction values, which are determined by adding a predetermined correction value to the values of the lower and upper dead points output by the program are output to the servo circuit. When the correction value is gradually increased, the insufficient measured distance is gradually reduced to L1, L2, L3 . . . , and at the time, the insufficient measured distance is equal to or less than a predetermined value, whereupon the increase in the correction value is stopped, and thereafter the chopping is effected in response to the output values. This is called a servo correction.
Further, when the lower dead point and the upper dead point are to be changed thereafter to the positions Zl 2 and Zu2, respectively, the coordinate values thereof are input through the program, an MDI, or the like. When these coordinate values are input, the numerical control apparatus once cancels the previously effected servo correction, again measures an insufficient distance caused by the newly instructed coordinate values, and starts a servo correction in accordance with the insufficient measured distance.
Nevertheless, a problem arises in that, since each time coordinate values are changed a servo correction must be canceled and restarted, and a transient or delay period of the servo correction T1, T2, . . . occurs each time the servo correction is canceled, some time is needed to stabilize the chopping width. Further, a fine adjustment of the lower and upper dead points must be effected by inputting coordinate values to the MDI, but this operation is time-consuming.